Metal oxide adsorbents, particularly silica-based adsorbents, are widely used in both domestic and industrial applications. For example, they are used in process and analytical chromatography to conduct very difficult separations and to produce products having very high levels of purity. Such materials are also used in food processing, for example, to decolorize and purify sugars. They are also frequently used as catalysts supports. Most of these applications involve separations and in all cases the relevant adsorbent characteristic, such as pore size, pore volume, surface area, etc., are very important.
Such materials have been made by spray drying silica sols, followed by sintering of the particles to adjust the pore volume. For example, U.S. Pat. No. 4,131,542 to Bergna et al. discloses a method for making low cost silica packing by spray drying an aqueous silica sol to form porous micrograins, acid washing the micrograins and then sintering the acid-washed grains to reduce surface area by 5-20%.
U.S. Pat. No. 5,128,114 to Schwartz discloses making high-strength uniform porous silica microspheres by spray drying a mixture of silica hydrosol and ammonium nitrate or urea and then sintering the particles without fusing the microspheres into agglomerates.
The foregoing prior art methods have in common that particle sphericity can be obtained by spray drying and that sintering is required to adjust parameters such as pore volume and pore size.
In addition, Japanese Kokai 61-174103 discloses a method for making porous spherical fine powders by mixing a colloidal solution of inorganic oxides and an inorganic oxide hydrogel or xerogel to form a slurry and spray drying the slurry in a stream of hot gas. Upon sintering the spray dried particles at 600 C. for 3 hours, particles in the range of 1-20 micrometers having a pore volume of 0.1-0.8 cc/g are produced. While the use of hydrogels is disclosed, only the use of xerogels is exemplified.
U.S. Pat. No. 4,010,242 to Iler et al. involves making uniform porous silica microspheres by coacervating a solution of silica hydrosol and urea-formaldehyde or melamine-formaldehyde polymer under polymerizing conditions to form microspheres, oxidizing the microspheres to burn off the polymer and then sintering the oxidized microspheres to reduce surface area.
U.S. Pat. No. 4,902,413 to Stout et al. discloses making uniform porous silica microspheres by coacervating a solution of silica hydrosol and urea-formaldehyde or melamine-formaldehyde polymer under polymerizing conditions to form microspheres and removing the polymer by solvent extraction, after which the particles are dried without sintering.
While the prior art methods of producing spherical particles of this type are effective in particular respects, many require extraction of organic adjuvants. Moreover, it is difficult to adjust the process parameters of these prior processes in such fashion that critical properties such as pore volume, sphericity, microporosity, etc., can be controlled reliably.